1. Field of the Invention
This invention relates to digital-to-analog converter (DAC) drive circuits, and particularly to DAC circuits that are connected to output operational amplifiers having a permissible input voltage range less than the circuit""s rail-to-rail differential.
2. Description of the Related Art
DACs that are configured to operate in the voltage mode, in which an output analog voltage that corresponds to an input digital signal is produced, typically have their outputs buffered by an operational drive amplifier. This type of design is used, for example, in the PM-7224 8-bit CMOS DAC described in the Analog Devices, Inc. Data Converter Reference Manual, volume 1, 1992, pages 2-267 through 2-278.
The basic circuit is illustrated in FIG. 1. A voltage mode converter DAC1 is shown with its output 2 connected to the non-inverting input of a buffer operational amplifier A1. The output 4 of A1 is tied back to the amplifier""s inverting input, thus providing a unity gain drive for a load 6.
A typical circuit used to implement amplifier A1 is shown in FIG. 2. The circuit is supplied by positive and negative voltage supply lines Vdd and Vss, respectively. The voltage supply levels are referred to as the circuit xe2x80x9crailsxe2x80x9d. With Vss tied to analog ground potential, the circuit can be operated with a single power supply Vdd, typically 12-15 volts. The circuit could alternately be operated from dual power supplies, such as +5 and xe2x88x925 volts for Vdd and Vss, respectively.
The amplifier""s output stage is shown as an NPN bipolar transistor Q1 that provides a low-impedance, high-output current capability. The emitter of Q1 is loaded with a current source I1, such as a 400 microamp NMOS current source referenced to Vss, while its collector is connected to Vdd. Sinking the I1 current into Vss allows the amplifier""s output to go directly to ground.
An input stage consisting of art  an NMOS transistor Q2 has its drain connected to Vdd, with another current source I2 sinking current from the source of Q2 to Vss; the Q2 source is also connected through a resistor R1 to the base of Q1. Transistor Q2 operates as a source follower, driving the resistor R1 and output transistor Q1.
The converter DAC1 by itself is a high impedance device; the operational amplifier A1 provides a buffer function to drive the load. However, proper operation of A1 generally requires that its input 8 from the DAC be 1 volt or more below the positive voltage supply level Vdd. This means that the voltage swing at the output of the DAC must be limited to the permissible input voltage range for the amplifier, and consequently also limits the output range for the overall circuit to a similar level. The circuit is thus limited to an output range less than a desired xe2x80x9crail-to-railxe2x80x9d voltage swing.
The present invention seeks to provide an improved DAC drive circuit that is capable of operating in a voltage mode with a full rail-to-rail output range. This goal is accomplished by dividing the DAC output by a factor that places it within the permissible input range for the op amp, and providing the amplifier""s output stage with a gain that allows the overall drive circuit to produce a rail-to-rail output if desired.
In a preferred embodiment the divider is implemented with an attenuation network, in the form of m dummy DAC bits, that is impedance matched to the DAC. The DAC, which is connected to receive an n-bit digital signal, thus has n+m bits. The dummy bits are connected as the DAC""s m most significant bits, and are always OFF. This produces a downscaling of the input signal range by a factor xc2xdm. The op amp is configured to produce a compensating 2m amplification, and includes a feedback circuit that is also impedance matched to the DAC. By setting m equal to 1, the DAC""s output range is divided by 2, with the op amp doubling the DAC""s output to restore a rail-to-rail output swing.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.